The objective of this proposal is to investigate the mechanism of action of the angiotensin converting enzyme (peptidyldipeptide hydrolase, E.C. 3.4.15.1). This enzyme catalyzes the hydrolysis of the dipeptide, His-Leu, from the carboxyl terminal of the decapeptide, angiotensin I, but is, in fact, a general peptidyldipeptidase with activity toward a broad range of oligopeptide substrates. In addition, it is believed to be a zinc metalloenzyme. Thus, it appears to share certain characteristics with two of the more extensively studied zinc metallopeptidases, bovine carboxypeptidase A and thermolysin from B. thermoproteolyticus. Hence, these will serve as models for structure-function investigations of converting enzyme. Detailed kinetic comparisons of these enzymes will be carried out in order to define those mechanistic features that they may have in common. Certain aspects of substrate specificity will be examined, a variety of peptide analogs will be tested for reversible inhibition, and the role of chloride ion as activator will be studied. Finally, mass spectrometry will be employed in order to investigate the exchange of O-18 between solvent and substrate. Such studies will be important in the identification of acyl enzyme intermediates in the catalytic process.